Servomotor



May 22, 1945; Do soN SERVO-MOTOR Filed Sept. 26, 1940 s Sheets-Sheet 1 lNVE/VTOR [dwardfiodson BY Oil/away ATTORNEYS E. DODSON May 22, 1945.

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'. lNl/[NTO/i [award Dadson By 9hm% ATTORNEYS Patented May 22, 1945 SERVOMOTOR Edward Dodson, Putney Heath, London, S. W. 15,

England Application September 26, 1940, Serial No. 358,415 In Great Britain January 3, 1940 (Cl. ll37139) 2 Claims.

The invention provides a hydraulic servomotor which includes a device normally overriden by the pressure fluid but operative to move the piston to a definite predetermined position, should the pressure of the fluid be released with the piston in a position unfavourable to the working of the apparatus to be operated by the servo- :motor; The device for moving the piston to the predetermined position on release of the pressure fluid may conveniently be constituted by a spring.

The invention is of particular, but not exclusive, application to remote acting throttle controls for internal combustion engines, more particularly aero engines. Fluid pressure may be absent in the servomotor either by reason. of a mechanical failure or because the engine which produces the pressure has not commenced to operate. When an aero engine is to be started up, there will be no pressure fluid to operate the servomotor unless a fluid pressure reservoir is provided, and unless the cowling is dismantled it is not possible to set the throttle valve at the starting position by hand. By employing the invention, however, it

may be arranged that should the pressure fluid in the'servomotor be released, with the throttle valve in the idling position, the piston of the servomotor will at once move the throttle valve into an open position suitable for starting.

One embodiment of the invention as applied to aremote acting throttle control for an aero engine will now be described in detail, by way of example, with reference to the accompanying drawings, in which Figure l is an elevation of the servomotor,

' partly broken away, Figure 2 is a section taken on the line 11-11 in Figure 1,

Figure 3 is a diagram illustrating the connections between the pilots throttle lever, the servomotor and the throttle valve, 7 a

Figure 4 is an elevation of the sender unit associated with the pilots throttle lever,

Figure 5 is a section on the line V-V in Figure '4, and

Figure 6 is a section on the line VI-VI in Figure 5.-

The scrvomotor consists of the usual cylinder Hi, piston ll, piston valve l2, swinging lever l3, pivoted at I! to a bracket I5 projecting from the housing of the motor and at 38 to a fitting 22 on the lower end of the piston rod 22, and follow up gear l6. I1 is an oil inlet port and I8 is an oil outlet port. The motor is controlled by compressed air admitted through a port l9 and operative on the upper surface of a diaphragm 20 connected to the piston valve l2. The action of the diaphragm 2% on the piston valve i2 is opposed by springs 37?. The pressure of the compressed air operating on the upper surface of the diaphragm 2b is varied in accordance with the position of the pilots throttle lever through the intermediary of a sender unit 39 illustrated in Figures 4-6; The sender unit 39 comprises a two-part casing containing a diaphragm it clamped between thetwo parts of the casing. The upper surface of the diaphragm M is exposed to atmospheric pressure and its lower surface is exposed to compressed air admitted to the lower portion 5! of the casing through an inlet it under the control of an inlet valve d2. An exhaust port it in the lower portion of the casing is controlled by an exhaust valve M. The valves d2, tit, respectively, actuated by tappets lii, it are attached to a rocking yoke M pivotally mounted at and pivoted at it to a lug 5U fixed to the diaphragm til. Downward movement of the diaphragm therefore tends to rock the yoke il clockwise as seen in Figure 3 and so open the inlet valve ii,

whilst upward movement of the diaphragm tends mined by the loading of a spring 52 acting on to open the exhaust valve M. Thus the diaphragm tends to maintain a predetermined pres: sure in the lower portion 5! of the casing, deter- The setting of the cam 55 and-consequently the air pressure maintained in the chamber 5i therefore depend on the setting of the pilots throttle lever. An outlet port 59 in the chamber communicates, by way of apipe 60, with the inlet port is and diaphragm 20 of the servomotor. The setting of the diaphragm 20 will therefore also correspond to the-setting of the pilots throttle lever. On movement of the diaphragm 20, the piston valve i2 is displaced in the appropriate direction opening ports for connecting one side of the cylinder Hi to the oil inlet l1 and the other to the oil outlet l8. Thus, supposing the ir pressure above the diaphragm 20 to decrease, the piston valve It. will be displaced upwardly,

permitting oil to flow from the inlet ll through ports 29, 30 to the top of the cylinder and from the bottom of the cylinder through ports 32, 33, 34 to the oil outlet IS. The piston II will therefore move down, swinging the lever l3 clockwise about its pivot l4 and lowering the piston valve l2 to its neutral position through the intermediary of the follow-up gear H5 in the well known manner. Conversely, if the pressure above the diaphragm increases, the piston valve l2 will move down, connecting the oil inlet I! through the ports 29, 33, 32 to the lower end of the cylinder, and the upper end of the cylinder through the ports 3|, 30, 34 to a passage 35 connected to the oil outlet l8. The connection between the passage 35 and the outlet I8 is not shown in Figure 2 because it lies above the plane of the section. It is however constituted by a bore formed in the wall of the servomotor. The piston II will therefore rise, returning the piston valve l2 to its neutral position through the intermediary of the follow-up gear l6 as before. The

lower diaphragm 36 is not exposed to air pressure but makes provision for self-alignment of the piston valve. It is necessary for the piston valve to respond to very small pressure differences, and therefore any side thrust on the valve must be avoided.

'The fitting 22 on the lower end of the piston rod 2| is connected to the throttle valve 6| by a rod 62 (see Figure 3), so that the throttle valve will take up a position determined by the, position of the piston and therefore corresponding'to the setting of the distant pilots throttle lever. The apparatus is shown in the drawings with the piston H in its lowermost position which corresponds to idling of the engine. Should the oil pressure be cut 01? under these conditions, the

a rod 25, carrying near its lower end a collar 26, l

and at its upper end a cap 21 forming an abutment for the upper end of the spring 23. Across the slot in the bulge 24 extends a slotted pin 28 having a flat on its lower surface which abuts against the top of the collar 26, the rod passing through the slot in the pin 28. When therefore the piston ll occupies a position near the lower end of the cylinder,.the rod 25 is pulled down by the pin 28, thus compressing the spring 23 which is insufficiently powerful to oppose the pressure of the oil. On release of the oil pressure however, the spring 23 will expand and raise the piston I i into a position corresponding to an open throttle. Owing to the lost motion connection between the rod 25 and the swinging lever l3, the spring 23 will not be compressed when the piston II is at the top of the cylinder, but should the pressure of the 'oil be cut off in this position the throttle will of course be sufiiciently open to permit of starting of the engine.

What I claim as my invention and desire to secure by Letters Patent is:

1. In a remote acting fluid-pressure-operated throttle control system for an internal'combustion engine, of the kind comprising a pressuresensitive device located in the vicinity of the throttle valve and responsive to changes in fluid pressure transmitted to said device from a disstance under the control of a distant throttleactuating member, a relay-valve operatively connected to the pressure-sensitive device, and a servomotor coupled to the throttle valve and operating under the control of the relay valve to take up a position determined by the fluid pressure acting on the pressure-sensitive device, the combination with the piston of the servomotor of a biasing device, arranged to be overridden by the fluid pressure in the servomotor on movement of. the piston into a position corresponding to idling of the engine but operative to move the piston into an open throttle position suitable for starting in the event of failure of' the fluid pressure in the servomotor, and a lost motion connection between the, biasing device and the piston which operates to relieve the biasing device from stress except when the piston approaches the idling position,

2. In a remote acting fluid-pressure-operated' throttle control system for an internal combustion engine, of the kind comprisin a pressuresensitive device located in the vicinity of the throttle valve and responsive to changes in fluid pressure transmitted to said device from a distance under the control of a distant throttleactuating member, a relay valve operatively conne'ctedto the pressure-sensitive device,'a servomotor operated "by liquid pressure created by operation of the engine, said motor being coupled to the throttle valve and operating under the control of the relay valve to take up a position 

